The influence of multiple temporal memories in the peak-interval procedure.

Abstract

Memories for when an event has occurred are used to anticipate future occurrences of the event, but what happens when the event is equally likely to occur at two different times? In this study, one group of rats was always reinforced at 21 s on the peak-interval procedure (21-only group), whereas another group of rats was reinforced at either 8 or 21 s, which varied daily (8-21 group). At the beginning of each session, the behavior of the 8-21 group largely lacked temporal control, but by the end of the session, temporal control was reestablished. When both groups were reinforced at 21 s, the patterns of responding were indistinguishable after subjects in the 8-21 group had experienced 13 reinforcement trials. Finally, the reinforcement times of previous sessions affected the 8-21 group, such that subjects were biased depending on the reinforcement time of the prior session. These results show that when the reinforcement time is initially ambiguous, rats respond in a way that combines their expectations of both possibilities; then they incrementally adjust their responding as they receive more information, but still information from prior sessions biases their initial expectation for the reinforcement time. Combined, these results imply that rats are sensitive to the age of encoded temporal memories in an environment in which the reinforcement time is variable. How these results inform the scalar expectancy theory, the currently accepted model of interval-timing behavior, is discussed.

Figure 1A shows the normalized response rate (y-axis) for the entire data set of the 8–21 subjects and 21-only subjects as a function of trial duration (x-axis), while (B) shows this same data with relative time (actual time/the averaged peak time) on the x-axis. A two-model fit was used to find the the number of reinforcement trials that had to be experienced before temporal control was re-established by the subjects in the 8–21 group on 8 and 21 second sessions. Peak trials that occurred before these points were excluded so that only data (normalized response rate) from trials occurring after these timepoints were compared to the 21-only group as a function of (Figure 1C) real time (x-axis) and (Figure 1D) relative time (x-axis).

The lever pressing behavior of the 8–21 subjects was different from the 21-only subjects early in the sessions; after experiencing several FI trials the behavior of these two groups overlapped; error bars represent the standard error of the mean. Figure 2B: the lever pressing behavior of the 8–21 subjects when reinforced at 8 seconds and 21 seconds was indistinguishable early in the session and diverged after experiencing a few FI trials.

The lever pressing behavior of the 8–21 subjects when reinforced at 8 seconds was dependent on the previous session; error bars represent the standard error of the mean. If they were reinforced at 21 seconds, they began the current session biased towards this time, and if they were reinforced at 8 seconds, they began the current session biased towards 8 seconds. Figure 3B: Like their behavior on days when the reinforcement time was 8 seconds, lever pressing behavior of the 8–21 subjects when reinforced at 21 seconds was dependent on the previous session. If they were reinforced at 21 seconds, they began the current session biased towards this time, and if they were reinforced at 8 seconds, they began the current session biased towards 8 seconds.